Electrical motor monitoring system for a domestic appliance

ABSTRACT

A drive system for an automatic washer or dryer having a rotatable drum about a horizontal axis. An induction motor drives the drum and is connected to and disconnected from a source of alternating voltage by a microprocessor to control the speed of the motor. The microprocessor senses the zero crossing of the alternating voltage and the zero crossing of alternating current flowing in the motor to determine the time to connect the motor to the alternating voltage. The microprocessor analyzes successive readings of the motor&#39;s back emf to detect undesirable load distributions and effect redistribution.

This is a continuation of application Ser. No. 182,509, filed Apr. 18,1988, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates in general to an appliance, such as anautomatic washer or dryer, and more particularly to an appliance havinga clothes holding drum which rotates about a horizontal axis.

A typical automatic dryer for drying articles such as clothes has anouter cabinet, a rotatable drum driven by a motor within the cabinet, afan for drawing air in through the cabinet and passing it to the drumand a discharge duct for exhausting the air from the drum to theatmosphere. Usually, dryers of this type have one or more electricalheating elements located in the inlet air duct to heat the air before itpasses to the drum. Alternatively, the heat is supplied from a source ofgas. Conventionally, dryers of this type have a main on/off switch andan adjustable timer so that a user can select any one of a range ofdrying times. A heating control switch ultimately varies the amount oftime power is supplied to the heating elements. Prior art automaticwashers also have a rotatable drum or washtub within a cabinet.

Modern automatic washers and dryers typically are microprocessorcontrolled and the number of actual controls which the user has accessto is less than the number of controls for older type dryers. Prior artcontrol circuits for dryers using a microprocessor typically haveinvolved complex circuitry. The present invention provides a simplersolution to the problem of providing a circuit in a microprocessorcontrolled dryer or washer for controlling the motor.

SUMMARY OF THE INVENTION

The present invention involves a drive system for an automatic washer ordryer having a drum rotatable about a horizontal axis. An inductionmotor drives the drum and is connected to and disconnected from a sourceof alternating voltage. When the zero crossing of the alternatingcurrent flowing in the motor is sensed, the back emf of the motor isdigitized via an A/D. Successive digitizations when processed, yieldvaluable motor loading information useful in correcting the drying orwashing process.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures in which like referencenumerals identify like elements, and in which:

FIG. 1 is a perspective view, partially cut away of an automatic clothesdryer utilizing the present invention;

FIG. 2 is a general block diagram of the circuit used in the FIG. 1dryer;

FIG. 3 is a graph of voltage and current waveforms in the FIG. 2circuit.

FIG. 4 is a more specific circuit diagram of the FIG. 2 circuit;

FIG. 5 is a more detailed graph of voltage and current waveforms in theFIG. 4 circuit; and

FIG. 6 is a graph depicting the drying time for different loads ofclothes in an automatic dryer.

FIG. 7 is a block diagram of an alternative embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention has general applicability but is mostadvantageously utilized in an appliance, an example of which is shown inFIG. 1. It is to be understood that the present invention also has usein an automatic washer, but the present invention will be described asused primarily in an automatic clothes dryer, which constitutes thepreferred embodiment.

A clothes dryer 10 has an outer cabinet 12 with an access port 14 in afront of the cabinet 12. Within the cabinet 12 there is provided aclothes tumbling drum 16 mounted for rotation about a horizontal centralaxis. The drum 16 is cylindrical in shape and has paddles 17. The drum16 is driven by a belt 19 which is connected to a motor 21 as is knownin the art.

The clothes dryer 10 is typically provided with a control arrangementsuch that an operator, by manually setting a control knob 18 andactivating a push to start switch (not shown) causes the machine tostart and automatically proceed through a desired drying cycle.

The clothes dryer 10 is provided with an inlet duct 20 which has a covergrill 22 out of which air flows after being heated by a heating element24 in the inlet duct 20. A blower housing assembly 26 is also providedand air from the drum 16 exits through a cover grill 28 through adischarge duct 30 and out to the atmosphere. Within the discharge duct30 a thermostat 32 is located and adjacent the thermostat is a biasheater 34. A lower motor (not shown) causes air to be pulled out of thedrum 16 thus causing the air to flow through the inlet duct 30. As theair exits the drum 16 it flows over the thermostat 32. The thermostat 32has a predetermined set point at which it will cause the heatingelements 24 in the inlet duct to turn off. For example, the thermostatmay be set at 75° C. The thermostat is heated by both the air flowingout of the drum 16 and by the bias heater 34. A microprocessor via acontrol circuit operates the motor 21 as well as the thermostat 32 andbias heater 34 to effect proper drying of a load of clothes.

The present invention is most advantageously utilized in the control ofan induction motor used in an automatic washing machine and/or anautomatic clothes dryer.

FIG. 2 is a schematic block diagram of an induction motor 100 having awinding 102 connected to an alternating voltage, V_(L), at terminal 104,and via a triac 106 to ground or the neutral of the alternating voltage.As is well known in the art, the motor 100 may be controlled by means ofa microprocessor 108 via a trigger boost circuit or control circuit 110which is connected to a gate G of the triac 106. The microprocessor 108causes a signal from the control circuit 110 to be applied to the gate Gof the triac 106 typically during each half cycle of the alternatingvoltage V_(L). Once the triac 106 is triggered into conduction, and acurrent flows through the winding 102, thus energizing the motor 100,the triac 106 will continue to conduct until the next zero crossing ofthe current. The amount of time after the zero crossing at which thetriac is re-triggered into conduction affects the speed of the motor100.

As shown in FIG. 3, the voltage V_(L) may be sinusoidal, having a zerovoltage level or a zero crossing at V_(zx). The current which flowsthrough the winding 102 of the induction motor 100 is out of phase withthe applied voltage V_(L) and when the motor is operating at a highspeed, may be represented for example by the "fast" curve I_(mf) andwhen the motor is operated at a low speed may be represented by the"slow" curve I_(ms). For a particular operation and speed of the motor100, for example, the triac 106 may be triggered into conduction atpoint in time T₁. At a later point in time T₂, corresponding to the zerocrossing of the current, the triac 106 will turn off. The zero crossingof the current through the winding 102 is referred to in FIG. 3 asC_(zx). The time from the zero crossing of the current C_(zx) until thefollowing triggering at time T₁ of the triac 106 may be a fixed value ormay be a variable value which is determined by the microprocessor fromother parameters.

The induction motor 100 has a speed which may be controlled bymaintaining the timing interval between the zero current crossing C_(zx)and the moment of triggering T₁. As explained above, the time intervalbetween the voltage zero crossing V_(zx) and the current zero crossingC_(zx) is shorter for a motor losing speed. In other words, the currentlags the voltage by a smaller phase angle amount and thus C_(zx) movescloser to V_(zx). When triggering is based on a fixed time interval fromC_(zx) the triggering also moves to the left in FIG. 3 for a motorlosing speed. This triggers the triac 106 earlier in time, which willmake the motor speed up. These two opposing conditions, therefore, causethe motor to seek equilibrium. It can be seen that this condition canoccur at every half line cycle.

As shown in FIG. 2, circuit block 112 senses the zero crossing of thesupply voltage V_(L) and provides an output signal, V_(zx), indicativeof this to the microprocessor 108. Circuit block 114 determines the zerocrossing of the current flowing through winding 102 from the voltageV_(T) across the triac 106 and produces the signal C_(zx) for themicroprocessor 108.

As shown in FIG. 4 the voltage zero crossing sensor 112 is connecteddirectly to the line voltage V_(L) at terminal 104. The line voltageV_(L) is connected via resistors and capacitor R1, R2 and C1 as shown inFIG. 3 to the base of a transistor Q1. As the voltage at V_(L) risesabove ground, Q1 becomes forward biased and turns on, pulling itsoutput, V_(zx) to ground. The current sensing block 114 has its inputconnected to receive the voltage V_(T) and when the triac 106 isconducting negative current V_(T) is negative which keeps transistor Q2in an off condition. The base of transistor Q2 is connected throughresistors R3 and R4 to the voltage V_(T). When the triac 106 goes intothe nonconducting state, the voltage V_(T) rises, thereby forwardbiasing Q2 which pulls its output C_(zx) to ground. Thus, the timebetween V_(zx) being pulled to ground and the time C_(zx) is pulled toground is a function of motor speed and loading. This information canthen be used by the microprocessor 108 to determine the time oftriggering triac 106 through the circuit 110. As stated above, the motor100 can be caused to keep a constant speed in consideration of changingload conditions or can be caused to accelerate or decelerate dependingupon the application. The microprocessor 108 is not described or shownin detail as there are many suitable microprocessors available which canbe easily programmed by one skilled in the art.

FIG. 5 shows a more detailed graph of the voltages in the FIGS. 2 and 4circuits. As was stated above, as long as there is sufficient currentflowing through the triac 106, the polarity of the voltage V_(T) acrossthe triac is always in phase with the current flowing through the triacand the winding 102. For example, when the motor-triac current flowsfrom V_(L) to N, V_(T) is greater than or equal to +1.6 volts. Thisbiases transistor Q2 and turns on Q2 thereby pulling C_(zx) low.Conversely, when the motor-triac current flows up from N to V_(L), V_(T)is less than or equal to -1.6 volts. Transistor Q₂ is off and C_(zx) isheld high. It is to be noted that both the positive current zerocrossings and the negative current zero crossings can be detected withthis circuit.

Zener diode Z₁ as shown in FIG. 4 prevents excessive power dissipationin the base of the transistor Q₂. During times when the triac 106 is inan off condition and the voltage V_(T) approaches the line voltageV_(L), excessive voltage levels which could stress the transistor Q2 arediverted through the Zener diode Z₁.

As can be seen in FIG. 5, the voltage V_(T) changes state between a nearzero level and a higher level at each zero crossing of the currentflowing through the triac 106 and the winding 102.

The voltage, V_(T), across the triac 106 provides certain informationwhich may be utilized by the microprocessor in operating the automaticdryer and/or washer. For example, if V_(T) is significantly less thanV_(L) because of the motor 100's large back EMF, this indicates to themicroprocessor that the motor is running. If the voltage V_(T) isapproximately equal to the voltage V_(L) because of a decrease in theback EMF of the motor, this result indicates to the microprocessor thatthe motor is in a locked rotor position. This is so because when theinduction motor is running there is always a certain amount of what maybe referred to is as a back EMF. This can be seen in FIG. 5 as adifference V_(EMF) between V_(L) and V_(T). Also, if the motor 100 isjammed, the back EMF of the motor will be small, which in turn meansthat V_(T) will be very large.

The circuit and method described above can be used to sense andredistribute an unbalanced load of clothes in either an automatic washeror an automatic dryer which has a rotation about a horizontal axis.Thus, the load in the appliance may be distributed evenly beforeaccelerating to a high speed. The voltage V_(T) off (see FIG. 5) variesas a function of rotor speed in an induction motor. The harder the motoris working, for example, when it is lifting an unbalanced load ofclothes, the slower the rotation and the closer to applied line voltageV_(T) off approaches. When variations in successive measurements ofV_(T) off exceed some threshold limit, an unacceptably balanceddistribution of the clothes load has been detected. In order to effectthe redistribution of this unbalanced load, a time is determined atwhich the speed of the rotating drum is to be suddenly slowed orsuddenly accelerated. Sudden slowing of the drum causes the clump ofclothes to begin to fall off one of the paddles that is lifting it.Since the items in this group of clothing are not all equally distantfrom the bottom of the drum towards which they are falling, the suddenincreasing of the surface speed to which the items are falling tends tospread out the items. Breaking and accelerating of the drum can becontrolled by the microprocessor 108. Since it is possible now to evenlybalance the clothes in a horizontal axis washer, higher spin rates arepossible as compared to prior art devices.

In another embodiment of the present invention, illustrated in FIG. 7,it is possible to predict the drying time of a load of clothes in anautomatic dryer. As shown in FIG. 7, the voltage V_(T) is connected to adiode D, and a resistor network R₅, R₆ is connected to an eight bitanalog-to-digital converter 120, which receives an input approximatelyequal to V_(T) /40. The output of the converter 120 is the valueV-TRIAC-OFF. This output is read by the microprocessor 108 andsuccessive readings are stored internally. The microprocessor 108accesses a memory 109 in which is stored the data for different dryingcurves.

Every load of clothes dries at a rate which is determined by its sizeand character. Larger loads take longer than small loads, cottons takelonger than synthetics, and bulky loads take longer than shears. Thegraph shown in FIG. 6 shows the time versus water retention fordifferent sizes and types of materials in loads. The microprocessormemory 109 contains data with respect to the rate of drying fordifferent types of loads. This data may be empirically prepared byexperimentally weighing the clothes every few minutes and graphing thewater retention versus time. From this data an extrapolation can be madeas to when the clothes should be dry. The slope of the graph is afunction of the rate that water is being removed. It has been found thatthese graphs follow a linear curve quite closely as the clothes aretumbled, for example, in a horizontal axis automatic dryer. When theheater coils in a dryer are cycled on and off, nonlinearities areintroduced into the curve of the graph. To finish drying a load ofclothes, an additional drying time period is added on according to theoperator's dryness selection. From the graph of FIG. 6 it can be seenthat loads of 3#, 6#, 9# and 12# take about 9 minutes longer to dry foreach 3# increment from a 65% retention level to a 15% retention level.

The present invention implements the above method to effect the dryingof loads as follows: From a stopped drum position, the voltage V_(T) isdigitized by converter 120 to provide a direct measurement of linevoltage and then the drum is started. When the clothes fall off apaddle, the motor will attain full speed. The voltage V_(T) across thetriac after the triac has commutated off will be at a minimum value, Vtriac-off-min, indicating maximum speed has been achieved. The clotheswill be lifted in a clump by a paddle until several tumbles haveoccurred and will slow the motor down in proportion to the weight of theload. When the motor has slowed to a minimum speed, the voltage V_(T)across the triac will be at a maximum value, V triac-off-max, indicatingmaximum lift. The magnitude of the difference between V-triac-off-maxand V-triac-off-min, adjusted for line voltage variations, will be afunction of the load weight.

The calculated difference is stored by the microprocessor as the initialweight of the load. The microprocessor next takes at least twosuccessive measurements of V-triac-off. If the first measurement ofV-triac-off-max minus V-triac-off-min is large, then the microprocessorcan assume the load contains much water and wait, for example, for 10minutes before stopping the drum and taking another measurement of loadweight. If the initial value of V-triac-off-max minus V-triac-off-min issmall, then the load contains less water and another measurement can betaken, for example, after 5 minutes.

Successive measurements of V-triac-off-max minus V-triac-off-min aretaken to determine data points that will define a graph of the rate ofchange of the percentage of water retention versus time. These can bematched with one of, for example, 16 different drying curves stored inthe memory of the microprocessor. From a look-up table and theoperator's selected degree of dryness desired, the time remaining untilthe load is dry can be predicted. As a check, V-triac-off-max minusV-triac-off-min will approach zero as the load becomes dry. This isbecause a dry load tumbles with much, much less clumping than a wetload, thereby applying a more constant loading to the motor.

In addition, the present invention can estimate time remaining todryness, and can detect an empty drum. Furthermore, the presentinvention can detect a broken belt, in which case the drum is notturning. Additionally, the present invention can detect a jammed orlocked rotor by detecting that the value V_(T) off exceeds apredetermined threshold value.

The invention is not limited to the particular details of the apparatusdepicted and other modifications and applications are contemplated.Certain other changes may be made in the above described apparatuswithout departing from the true spirit and scope of the invention hereininvolved. It is intended, therefore, that the subject matter in theabove depiction shall be interpreted as illustrative and not in alimiting sense.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method for dryingloads of clothes in an automatic appliance having a drum rotatable abouta horizontal axis, comprising the steps of:providing an induction motormounted within the appliance; providing an alternating voltage/currentsource operatively coupled to said motor; linking said motor to saiddrum; sensing an alternating current flowing in said motor and providinga current signal indicative of a zero crossing of said alternatingcurrent; sensing said alternating voltage and providing a voltage signalindicative of the zero crossing of said alternating voltage; connectingsaid motor to said source of said alternating voltage at a point in timeT1 and disconnecting said motor at a next zero crossing of saidalternating current; providing a trigger from said voltage signal andsaid current signal for connecting said motor to said source; providingsaid trigger signal at said point in time T1 after receiving saidcurrent signal indicative of a zero crossing of said alternatingcurrent; digitizing a voltage across a means for connecting anddisconnecting said motor to and from said voltage source, whichdigitized voltage represents said motor's back emf; storing a pluralityof said digitized voltages and deriving therefrom several parameters,one of which determines a time interval between said point in time T1and a last previous zero crossing of said alternating current; from astopped drum position digitizing said voltage across said means forconnecting to provide a direct line voltage value; starting rotation ofsaid drum; digitizing a minimum value, V triac-off-min. of said voltageas said drum attains full speed; digitizing a maximum value, Vtriac-off-max, of said voltage as said drum attains full speed;digitizing a maximum value, V triac-off-max, of said voltage as saiddrum reaches a minimum speed; determining a difference value between Vtriac-off-max and V triac-off-min.; adjusting said difference value forany variations in said direct line voltage value, said difference valuebeing a function of the weight of the load of said clothes; storing saiddifference value; digitizing at least two successive values atpredetermined time intervals of V triac-off-max and determiningadditional difference values from V triac-off-max less V triac-off-min.;forming a calculated curve of the rate of change of the percentage ofwater retention versus time from said differences values; and matchingsaid calculated curve to a plurality of stored data curves foridentifying the time remaining until the load of clothes issubstantially dry.
 2. The method described in claim 1, wherein saidmethod further comprises the steps of:continuing digitizing said voltageand determining further difference values between V triac-off-max and Vtriac-off-min; and determining that the load of clothes becomes drier assaid difference values approach zero.